The viruses in flu shots are killed during the production of the vaccine, which means they cannot cause infection. The vaccine batches are then tested, with a group of people randomly assigned to get either the vaccine or salt water.

Still, the CDC reports that some people feel bad after a flu shot. Soreness at the injection site is one reason, but it usually dissipates within two days. It’s caused by the immune system making antibodies to the killed viruses in the vaccine that help a person fight off the flu.

Keep in mind that the vaccine does no kick in for two weeks from the time of injection, so it is still possible to contract the virus in that time. Also, the flu shot is not guaranteed to work, especially among the elderly and those with weak immune systems. But even among these high-risk groups, the vaccine can prevent complications.

2. It is better to get the vaccine later in the season to limit the risk that its efficacy will wane.

No. The shot lasts an entire flu season, except for some children who may need two doses. The CDC recommends that all people older than 6 months get a flu vaccine.

This is false. According to the American College of Obstetricians and Gynecologists, the flu vaccine is “an essential element of prenatal care” and is recommended for all pregnant women who are at high-risk for flu complications including pneumonia, infections and dehydration.

4. I’ve had the flu before and was fine, so I don’t have to worry about getting vaccinated.

Just because you’ve had the flu before and were able to beat the virus does not mean that you will be as lucky next time. According to the CDC, seasonal flu exacts a bigger toll in some years than in others: Between 1976 and 2007, the flu was linked to a low of 3,000 to as many as 49,000 fatalities in the United States, with more than 200,000 hospitalizations.

There are two main reasons: The viruses that circulate in one year may differ from those that circulate in another. And, of course, people change from year to year, meaning that your response to a viral infection one year may not be the same as your response in another.

5. The flu shot doesn’t work

It doesn’t work all the time, but it does confer some level of protection. For example, the CDC says preliminary data for the 2010-2011 season show that it was about 60% effective for all age groups combined, and studies for earlier years found protection rates of up to 90%.